Experts aren't sure why some illnesses travel in pairs, but knowing your risk will help you stay safe.

For years, doctors have observed that patients with one illness may be stricken by another condition that's seemingly unrelated and sometimes more serious, known as a "shadow disease." One of the most well-known is the connection between migraines and heart attack or stroke. Now researchers are uncovering even more linked ailments and zeroing in on why they appear to travel in pairs.

Studies show that these couplings occur for different reasons. In some cases, one disease creates damage that causes the second illness. In others, troublesome genes or poor health behaviors, such as smoking or lack of exercise, trigger one problem, then the other. Being alert to the following dangerous connections can help you avoid the shadow disease or get early diagnosis and treatment, leading to a better outcome.

If you regularly suffer from migraine pain (especially if you develop auras, which are visual or sensory phenomena that accompany the headache), your doctor has probably warned you about your susceptibility to heart attack or stroke. Now, thanks to recent findings, experts better understand which cardiac ailment is more likely to occur for any given migraine sufferer.

Frequency matters. If you have fewer than one migraine a month, you're 50% more likely to have a heart attack than nonsufferers. If migraines strike at least weekly, you have 3 times the risk of stroke, compared with those who don't have this problem, says study coauthor Tobias Kurth, MD, an assistant professor at Harvard Medical School.

Protect yourself: Unfortunately, existing research has not yet found that preventing migraines has the effect of lowering stroke or heart attack odds. However, by keeping your cardiovascular system as healthy as possible, you diminish your chance of a cardiac event, according to the National Stroke Association. To do this, control known hazards, such as high cholesterol and obesity, via diet and exercise. You should also quit smoking and limit alcohol intake (no more than one drink a day for women, according to the American Heart Association.) (Read Problem Solved: Migraines for more causes and cures.)

In 2007, a huge, 12-year French study confirmed that women with endometriosis (in which tissue similar to the uterine lining grows outside the womb) are 62% more likely to suffer from melanoma. Researchers are unsure why endometriosis and the deadly skin cancer sometimes travel together, but one possibility is a genetic defect that triggers both conditions.

Protect yourself: If you have endometriosis, ask your doctor to scan your skin for melanoma, advises Jeffrey P. Callen, MD, a professor of dermatology at the University of Louisville School of Medicine. You can also do a self-check of your moles. If anything suspicious turns up, be sure to have it biopsied. Don't panic, though. "When diagnosed very early, melanoma is nearly 100% curable," Dr. Callen says. (Don't know what to look for? Check our list of skin cancer warning signs.)

Doctors have long wondered how high blood pressure and diabetes are related, because the two often appear together, especially in obese patients. Now, after following 38,000 midlife women for 10 years, researchers at Brigham and Women's Hospital and Harvard Medical School report that constantly elevated blood pressure, or hypertension, doubles your risk of developing diabetes, regardless of your BMI. More surprising, the risk of diabetes goes up if your blood pressure increases over time—even if it stays under the hypertension threshold.

The two problems may have a common cause, says study author David Conen, MD, a research fellow at Brigham and Women's Hospital. Potential culprits include high levels of inflammation in the body, he says, or a malfunction in the blood vessels' inner lining, allowing blood cells to leak into surrounding tissue and damage it.

Protect yourself: If you have mild hypertension or are at risk for this problem, get tested for diabetes. You can help prevent onset of diabetes with lifestyle changes: Increase your physical activity, lose excess weight, limit salt, and stub out those cigarettes for good, suggests Dr. Conen. (Learn more about reversing your risk with our Let's Outsmart Diabetes campaign.)

The rough, itchy patches of psoriasis are more than uncomfortable and unsightly: They may increase your odds of a heart attack, concludes a study that followed half a million people for 5 years. The risk of a cardiac emergency was related to the severity of the psoriasis, the researchers determined; serious cases of the skin ailment could mean a more than doubled heart attack risk.

"The out-of-whack immune system that triggers the psoriasis may also cause inflammation that infiltrates the arteries of the heart," explains study author Joel M. Gelfand, MD, an assistant professor of dermatology at the University of Pennsylvania.

Protect yourself: Will controlling the skin disease diminish the inflammation that can cause a heart attack? The idea makes sense but must be confirmed with further research, says Gelfand. You can also consider talking with your doctor about new psoriasis medications, including Amevive (generic name alefacept). They're made from living sources, much as vaccines are. They may work, even if past treatments failed. In the meantime, Dr. Gelfand suggests keeping your heart healthy with diet and exercise. (And also with these 12 DIY Psoriasis Solutions.)

Metabolic syndrome is a serious health condition associated with coronary heart attacks, diabetes, and even early death. You have it if you've got at least three of these five traits: excess abdominal fat, high blood triglycerides, low HDL (which is the good cholesterol), high blood pressure, and impaired glucose tolerance.

Now research reveals that metabolic syndrome could also be behind the rising rate of kidney stones. Your odds of developing them go up by 54% if you have two of the above traits; with three symptoms, your risk hits 70%, says the research.

Protect yourself: Obesity is a key player in both metabolic syndrome and kidney stones, perhaps because overweight people are likely to consume excess protein and sodium, which may cause the painful crystals to develop, says study coauthor Bradford Lee West, MD. Trimming your waistline reduces one of the metabolic syndrome traits and may diminish your chances of getting kidney stones, he says.

Studies reveal a striking connection between asthma and psychological problems, including depression and anxiety. In 2004, the CDC announced that 18% of patients with asthma report mental distress. "This link has been debated by scientists for about 20 years, but in the last half decade it has become more accepted," says Bruce G. Bender, PhD, who is a professor of psychiatry at the University of Colorado Health Sciences Center.

Research on military veterans with post-traumatic stress disorder also confirmed the connection. Veterans with the greatest number of PTSD symptoms were more than twice as likely to have asthma as those with the mildest cases, reports study coauthor Renee D. Goodwin, PhD, an associate professor of epidemiology at Columbia University. Speculates Bender: "One condition may lead to the other. Breathing difficulties may cause anxiety or even depression. Or psychological problems may make asthma worse."

Protect yourself: If you know you have either asthma or mental health problems and suspect you have the other, get tested for it. Seek help for any confirmed illnesses so you don't find yourself in a downward spiral, with each condition exacerbating the other one.